A tightening torque detection unit capable of detecting a tightening torque when tightening a tightening member such as a bolt or a nut has been proposed. The tightening torque detection unit is used either installed in a tightening apparatus or removably installed in a power transmission mechanism of a tightening apparatus.
Such a tightening torque measuring unit comprises, as shown in FIG. 5, a torque sensor 12 such as a strain gauge, an amplifier 14 connected to the torque sensor 12, an A/D converter 16, a microprocessor (or MPU) 20, a display means 30 that displays a measured output torque value, and as necessary a storage means 32 that stores the measured torque value.
The amplifier 14 either amplifies a torque output received from the torque sensor 12 with an adjusted gain 15 or corrects the torque output with an offset 16a, and then transmits the amplified or corrected torque output to the A/D converter 16, in which the torque output is then converted from analog to digital and transmitted to the microprocessor 20.
The microprocessor 20 includes a torque conversion means 28a that converts a digital signal input from the A/D converter 16 into a torque value, and a peak torque detection means 26 that detects a peak torque value. The peak torque detection means 26 detects a peak torque value with a converted value obtained by the torque conversion means 28a so as to display the peak torque value on the display means 30 or store it in the storage means 32. See, for example, JP 2007-111797A.
Conventionally, the output from the torque sensor 12 has been amplified with a constant gain, irrespective of the tightening direction of the tightening apparatus. The amplification should ideally be performed so as to provide a direct proportional relationship between the output (mV) from the torque sensor 12 and the measured torque value (Nm). However, if the amplification is performed with a constant gain irrespective of the tightening direction of the tightening apparatus, the measured output torque value in either a clockwise or counterclockwise tightening direction tends to deviate from the actual value as the output (mV) of the torque sensor 12 increases, due to the characteristics of the torque sensor 12, the circularity of a portion on which the torque sensor 12 has been installed (e.g., the circularity of a shaft or a cylindrical body on which the torque sensor has been installed), or the like.
Thus, in order to improve the accuracy of the measured torque value in either one of the rotational directions, as shown in FIG. 6, such amplification has been performed so as to reduce an error between the introduced torque value (or actually applied torque value) and the measured torque value in either one of the rotational directions (the clockwise direction in FIG. 6).
The problem in this case is that, as to the other rotational torque (the counterclockwise direction in FIG. 6), an error between the measured value and the actual value will increase as the torque value increases.
An object of the present invention is to provide a tightening torque measuring unit capable of reducing an error as soon as possible in either a clockwise or counterclockwise tightening direction.